The present invention relates generally to conformal antennas and, in particular, to conformal microstrip antennas and to methods for mounting such antennas to surfaces with small radii of curvature for reliable operation.
The microstrip antenna, including its radiator and transmission line feed, is a well-known structure. In its simplest form, the microstrip radiator patch may simply be a square or rectangular metal conductor fed at one of its edges by an integral microstrip transmission line. This shaped transmission line/radiator structure is typically supported a short distance above a ground plane by a dielectric sheet or layer having a thickness substantially less than one-fourth wavelength at the intended operating frequency of the antenna (e.g., generally on the order of one-tenth wavelength or less). The resonant dimension of the radiator patch is typically chosen to be one-half wavelength, thus providing a pair of radiating slots between opposed edges (e.g., transverse to the feedline) and the underlying ground plane. The transverse or non-resonant dimension of the radiator is typically chosen, at least in part, as a function of the desired relative radiated power. If the non-resonant dimension is on the order of one wavelength or more, multiple feed points are generally provided (e.g., via a corporate-structure feed network). Such microstrip radiators may also be arrayed with corporate or other structures of microstrip feedlines integrally formed and connected therewith.
Conventional microstrip antenna structures are conveniently formed by photochemical-etching processes similar to those used in the manufacture of printed circuit boards. A microstrip antenna assembly is formed, typically, from flat printed circuit board material, that is a dielectric sheet material with a thin layer of conductive metal, such as copper, being adherent on both sides of dielectric sheet material. One conductive layer typically forms the ground plane or reference surface, and conductive metal is removed from the other layer by chemical etching to form the very thin microstrip radiator and interconnected transmission line structure as shaped conductive metal patches on the resulting dielectric sheet. The thickness of the entire antenna assembly, consisting of the dielectric sheet and the thin, conductive, metal layers, is on the order of one-thirty second (1/32) of an inch to two-tenths (2/10) of an inch. The thickness of the etched radiator and transmission line structure is commonly on the order of about 1.4 mils.
In microstrip antennas, the thickness of the dielectric sheet between the ground plane and the metal patches that act as the antenna radiators is selected based upon the bandwidth over which the antenna must operate. The bandwidth of microstrip antennas is proportional to the thickness of the dielectric substrate, as indicated by the following formula: EQU BW=4f.sup.2 (t/1/32)
where BW is the in megahertz for VSWR&lt;2:1, f is the frequency in gigahertz, and t is the board thickness in inches. Since the substrates are generally very thin in terms of the wavelengths (i.e., very much smaller than 1/4 wavelength), the bandwidth is usually narrow. The efficiency of the microstrip antenna is also affected by the thickness of the dielectric substrate. With thickness that is too small, the conductance across the dielectric yields excessive dielectric losses. Because of their contribution to antenna power loss, lossy dielectrics, such as epoxy fiberglass, are frequently avoided in microstrip antennas. The exact value of t has been generally determined by the commercially available printed circuit board thicknesses available from suppliers. Preferred materials include expensive Teflon-fiberglass boards of the type available from 3M, Rogers Corporation, Keene Corporation, and the like.
A desirable characteristic of microstrip antennas is their capability of being conformed to and mounted upon the curved surfaces. Because microstrip antennas may be conformed to curved surfaces, and provide a low profile (which reduces turbulence effects), and because microstrip antennas are able to withstand shock and vibration, they are often mounted on external; curved surfaces of airplanes, missiles, instrumented artillery shells, and other aircraft and projectile systems. Typically, mounting is accomplished by initially fabricating the antenna assembly as a flat sheet of dielectric material having microstrip radiation and transmission line elements formed on or otherwise adhered to one surface, and a ground plane adhered to the opposite surface. The assembly as a whole is then deformed or bent to conform to the curved surface on which the assembly is to be mounted and then secured to the surface.
Generally, the mounting surface is of convex shape; and the antenna assembly is deformed so that the microstrip antenna element will be on the outer convex surface thereof when the assembly is mounted to the surface. The concave surface of the deformed assembly is generally bonded to the mounting surface.
The above-described mounting procedure has not, however, proven to be fully satisfactory. As the microstrip antenna assembly is bent about small radii, the outer convex surface is placed under a substantial tension and must stretch, causing adjacent areas thereon to pull apart from one another. The stretching forces are transmitted to the antenna elements on the surface, tending to pull the antenna elements apart. Because of the thinness and fragile nature of the antenna (it is usually formed of relatively soft copper), the stretching forces can cause the antenna element to crack and can result in breaks in the conductive antenna elements, greatly reducing the effectiveness of the antenna structure. In addition, the stretching forces can cause loss of adherence between the antenna elements and the substrate, resulting in their separation from the underlying dielectric sheet. These unadhered antenna elements cannot reliably withstand the tremendous aerodynamic forces imposed on them by the projectile or aircraft upon which the antenna element is mounted as it moves through the air at high speed, and the unadherent antenna element can be torn away from the dielectric sheet and the microstrip antenna will be destroyed.
The exposure of a conformal microstrip antenna to such damage increases as the radius of curvature of the mounting surface decreases and as the thickness of the dielectric antenna substrate increases; and with the prior mounting techniques, it has been difficult to conform an antenna assembly onto a curved surface having a radius of curvature of less than about four inches without significant danger of substantial damage to the antenna element.